Paper or paperboard substrates used for printing and packaging are generally required to have good optical properties, excellent smoothness and excellent printability. Additionally, strength and stiffness are required such that the substrates can pass smoothly through high-speed printing and converting machines without breaking or jamming. High stiffness is necessary for maintaining the structural integrity of paperboard products during filling and in subsequent use.
Stiffness has a close relationship to the basis weight and density of the substrate. For a given caliper (thickness), the general trend is that stiffness increases as basis weight increases. However, if one increases basis weight to improve stiffness, more fiber must be utilized, adding to cost and weight.
In addition to the mechanical properties of stiffness and strength, paper or paperboard substrates that will be printed must have a required level of gloss and smoothness. One of the primary means for obtaining smoothness in a substrate is to calender the substrate during production. Calendering causes a reduction in caliper, which typically results in a corresponding reduction in stiffness. This is especially the case with the process of wet stack calendaring. Wet stack calendering requires a rewetting of a sheet that had been previously dried to about 5 percent moisture or less. The now rewetted sheet is passed through a calendering device having two or more rolls. The fiber network is compressed due to the pressure exerted by the rolls. The rewetting of the substrate makes the surface fibers more easily compressed and allows for more aggressive smoothness development. However, this compression densifies the sheet such that product manufactured using a “wet finish” process can have up to a 25% increase in its density after passing through the wet stack calender.
Alternately, manufacturers have attempted to smooth the surface of paperboard by coating the entire surface of the paperboard with a basecoat comprised of various pigments such as clay, calcium carbonate and titanium dioxide and then overcoating this base with a second and sometimes even a third coating, generally referred to as a topcoat. Typically, the more pigment (in the form of pigmented coatings) applied to the surface, the better the resulting smoothness. However, the use of relatively high quantities of pigments usually increases the cost and weight of the paper or paperboard.
The relationship between stiffness and smoothness is generally inversely proportional for a given amount of fiber per unit area. It would be desirable to be able to produce a finished paper or board having a smooth surface that was developed without the need for densification, thereby maintaining maximum thickness with the minimum cellulose fiber usage.